Epigenetic chromatin silencing is used by eukaryotic organisms to stabilize structural regions of the chromosome or prevent gene expression. A conserved feature of silent chromatin formation is the spreading of repressor complexes along the chromatin fiber by a mechanism that involves the self-association of histone- binding proteins. Sir3, a component of the silent information regulator (SIR) complex in budding yeast, binds to deacetylated histone tails and can form oligomers in vitro. I propose to study the driving forces of silent chromatin spreading by focusing on the role of Sir3 self-association. I will determine the domains of Sir3 that contribute to self-association in vitro and examine how self-association is modulated by other Sir proteins, histones, and small-molecule cofactors. In parallel, I will characterize Sir3 mutants that have a dominant- negative effect on silencing in vivo. As a longer-term goal, I will also work towards reconstituting the step-wise formation of silent chromatin on nucleosomal arrays in vitro. A full reconstitution of this system will be invaluable for testing the conclusions from the earlier aims of this proposal, as well as many other theories based on genetic studies. The physical interactions that drive silent chromatin spreading are also likely to contribute to maintenance and epigenetic inheritance of these regions. [unreadable] [unreadable] [unreadable]